The results of an experimental investigation are presented for the cross-flow past a pair of staggered circular cylinders, with the upstream cylinder being subject to forced harmonic oscillation transverse to the flow direction. Flow-visualization of the wake-formation region and hot-film measurements of the wake spectra are reported. Experiments were conducted in a water tunnel for Reynolds numbers, based on upstream velocity, U, and cylinder diameter, D, in the range 1440 < Re < 1680. Results are presented for the case where the longitudinal separation between cylinder centres (for the mean position of the upstream cylinder) is L/D = 2.0, with the transverse separation being T/D — 1.0. As shown by Sumner et al. this configuration corresponds to either the gap vortex pairing and enveloping or gap vortex pairing, splitting and enveloping regimes. The results show that the oscillation of the upstream cylinder causes considerable modification of the flow patterns and regimes compared to what is obtained when the cylinder is fixed. In particular, depending on the frequency of oscillation of the upstream cylinder, sub- and super-harmonic resonances are obtained between the vortex formation frequency and oscillation frequency, as well as the usual fundamental lock-in. These resonances and accompanying wake regimes are examined in detail in the paper.
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